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Altera_Forum
Honored Contributor
Honored ContributorUnnecessary instructions
Hello, it's my first post. I have question about GCC generated code. We can find several unnecessary instructions. --- sample code --- unsigned short uh; uh = IORD_16DIRECT(base,ofst)...
Altera_ForumHonored Contributor
Honored ContributorUnfortunately the "extra" instruction after the ldhuio is not due to the macro.
The gcc compiler shipped with Nios II 1.0 and Nios II 1.0.1 isn't very good about understanding that the 8 and 16 bit load unsigned instructions (I/O or normal) zero out the top bits and so it puts in "extra" andi instructions when not needed. I've reported this to our compiler engineer as an enhancement request. I believe this will be fixed in Nios II 1.1 (due out late November) when we upgrade to a new version of gcc. As for the warning in the documentation about not using bit 31 to bypass the data cache, I had them put that in to help make it easier for a possible future MMU option for Nios II. Being the CPU architect, it's my job to plan ahead.
